In a back light shooting condition under an open-air clear sky, an object luminance range within a scene (hereinafter, simply referred to as a “luminance range”) is widened. When the object having a wide luminance range is shot using a digital camera, the resulting scene may not be suitable for a dynamic range recordable by a photographic imaging system or an image signal processing system. In this case, a so-called shadow phenomenon in which an image is blocked up occurs in a dark portion of an image. Similarly, a so-called highlight phenomenon in which an image is blown out occurs in a bright portion of an image.
As a technique for avoiding such a phenomenon, there is known a high dynamic range imaging technique (hereinafter, referred to as a “HDR technique”). In the HDR technique, a shooting is performed several times for an identical scene by changing a shutter speed so that a plurality of image data are obtained with a different amount of exposure. Then, a synthesis processing is performed such that a pixel value of the image data obtained with a relatively large amount of exposure is used in an area where a shadow phenomenon may occur in an image, and a pixel value of the image data obtained with a relatively small amount of exposure is used in an area where a highlight phenomenon may occur. As a result, it is possible to obtain an image in which a gray scale is appropriately reproduced from a dark portion to a bright portion.
Japanese Patent Application Laid-open No. 06-141229 (JP06-141229A) discloses a technique of obtaining two or more images with a different electric charge storing time, adding the two or more images by applying a weight depending on a signal level of each image, and compressing the obtained signal level having a wide dynamic range to a reference level.
In the technique disclosed in Japanese Patent Application Laid-open No. 06-141229, it is necessary to increase a bit width (bit depth) when a plurality of signals are synthesized. For this reason, a necessary hardware size increases. In order to address such a problem, Japanese Patent Application Laid-open No. 2004-266347 (JP2004-266347A) discloses a technique of suppressing increase of the bit width (number of bits) of the image signal by performing a process of non-linearly compressing a high level part of the image signal and then synthesizing a plurality of images with a predetermined weight.